The reduced ability of the aging immune system to mount adaptive immune responses compromises the efficacy of vaccinations and increases the morbidity from infections. Adaptive immune responses to exogenous or endogenous threats rely on a diverse T cell repertoire and the rapid activation and expansion of an antigen-specific T cell population and acquisition of effector functions. In studying the effect of age on CD4 T cell function, we have identified a decline in miR181a as a characteristic hallmark of T cell aging. Our studies so far have focused on the dual-specific phosphatase (DUSP) 6 which is repressed by miR181a and therefore reciprocally increases with age. DUSP6 calibrates the T cell receptor activation threshold at which stimulation is translated into a productive signal. Increased DUSP6 contributes to the lowered sensitivity of elderly T cells to respond. miRNAs function by repressing the translation of sets of genes, frequently belonging to related pathways. The current proposal is based on the hypothesis that the decline in miR181a and the co-regulated miR181b is of broad importance to understand T cell aging and has consequences that go beyond increased DUSP6 activity. In addition to DUSP6, we will focus on SIRT1, BCL-2, and TCL1. What DUSP6 and SIRT1 have in common is that they control negative feedback loops in T cell activation. SIRT1, BCL-2 and TCL1 are critical components of T cell survival pathways. The decline in miR181a/b therefore results in a T cell phenotype that favors cellular longevity and quiescence at the expense of activation and effector function. In Aim 1, we will examine the epigenetic, transcriptional and post-transcriptional mechanisms that control miR181a/b expression. The objectives of these studies are to understand what drives the decline in miR181a/b with age and to identify means to upregulate expression. Aim 2 will examine the influence of age on the expression of SIRT1, BCL-2, and TCL1 in T cell subsets and determine whether age-related changes in protein expression are caused by the degree of miR181a/b expression and can be reversed by miR181a/b overexpression. In Aim 3, we will examine the functional consequences of miR181a/b loss, and we will determine whether they can be attributed to the overexpression of DUSP6, SIRT1, TCL1 or BCL-2.